The initiation of blood coagulation through the factor VII-tissue factor pathway is not only important for normal hemostasis, but is involved in pathological thrombosis as well. The cellular expression of tissue factor has been correlated with infectious, inflammatory, and neoplastic disease, and may be responsible in part for the thromboembolic complications of not only these illnesses, but also the venous thrombosis and pulmonary embolism occurring in otherwise normal individuals. Plasma contains an endogenous inhibitor of the factor VIIa/tissue factor complex, termed Tissue Factor Pathway Inhibitor (TFPI). One form of TFPI is bound to the lipoproteins in plasma, whereas another form is released into plasma (presumably from the endothelium) following the in vivo infusion of heparin. At physiologic concentrations, TFPI requires the presence of factor Xa to express feedback inhibition of the factor VIIa/tissue factor complex. We plan to extensively characterize lipoprotein-associated and heparin-- releasable forms of TFPI and investigate structure/function relationships in the TFPI molecule vis a vis its lipoprotein association, its inhibition of factor Xa, its inhibition of factor VIIa/tissue in the presence and absence of factor Xa, and the enhancement of its inhibitory activity by heparin and other glucosaminoglycans. Additional studies will investigate the mechanism of endogenous proteolysis of TFPI and the manner by which TFPI is associated with endothelial cells. Finally, to test the role of TFPI in a revised hypothesis of coagulation, we will determine whether the endogenous inhibition of TFPI ameliorates bleeding in dogs with hemophilia. The experiments are designed to provide information, now lacking, concerning the physiology of TFPI and its regulation of tissue factor-induced coagulation. The results should enhance our understanding of both normal hemostasis and pathological thromboembolism.